(ITS#3492) Backport 2.3 checkpoint behaviour to 2.2

Full_Name: Dave Horsfall
Version: 2.2.17
OS: FreeBSD 4.10
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (192.65.182.30)
The checkpoint behaviour in 2.3 was modified to checkpoint every "N" minutes
regardless of the time of last update. The documentation for 2.2 implies that
this is the case, but it is not; it would seem preferable to have the "obvious"
behaviour. Please consider this as a request for enhancement to implement the
2.3 behaviour in 2.2.